Fair Value Measurement and Fair Value of Financial Instruments (Carrying and Fair Values per the Fair Value Hierarchy of Certain Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Financial assets:
Cash and cash equivalents	$ 2,218,787	$ 2,174,592	$ 1,736,749	$ 1,878,503
Interest-bearing deposits with banks	400,900	398,422
Resale agreements	1,035,000	1,050,000
Restricted equity securities, at cost	73,729	73,521
Loans held-for-sale	3,114	85
Loans held-for-investment, net	30,900,144	28,688,590
Branch assets held-for-sale	0	91,318
Financial liabilities:
Short-term borrowings	56,411	0
FHLB advances	325,596	323,891
Repurchase agreements	50,000	50,000
Long-term debt	156,770	171,577
Carrying amount of repurchase agreements eligible for netting against resale agreements	400,000	400,000
Gross repurchase agreements	450,000	450,000
Carrying Amount
Financial assets:
Cash and cash equivalents	2,218,787	2,174,592
Interest-bearing deposits with banks	400,900	398,422
Resale agreements	1,035,000	1,050,000
Restricted equity securities, at cost	73,729	73,521
Loans held-for-sale	3,114	85
Loans held-for-investment, net	30,900,144	28,688,590
Branch assets held-for-sale		91,318
Mortgage servicing rights	7,861	7,771
Accrued interest receivable	136,932	121,719
Financial liabilities:
Demand, checking, savings and money market deposits	24,928,338	25,974,314
Time deposits	8,700,786	5,640,749
Branch liability held-for-sale		605,111
Short-term borrowings	56,411
FHLB advances	325,596	323,891
Repurchase agreements	50,000	50,000
Long-term debt	156,770	171,577
Accrued interest payable	21,579	10,724
Estimated Fair Value
Financial assets:
Cash and cash equivalents	2,218,787	2,174,592
Interest-bearing deposits with banks	400,900	398,422
Resale agreements	1,002,552	1,035,158
Restricted equity securities, at cost	73,729	73,521
Loans held-for-sale	3,114	85
Loans held-for-investment, net	31,003,048	28,956,349
Branch assets held-for-sale		94,245
Mortgage servicing rights	12,077	11,324
Accrued interest receivable	136,932	121,719
Financial liabilities:
Demand, checking, savings and money market deposits	24,928,338	25,974,314
Time deposits	8,730,602	5,626,855
Branch liability held-for-sale		643,937
Short-term borrowings	56,411
FHLB advances	335,800	335,901
Repurchase agreements	89,106	104,830
Long-term debt	162,566	171,673
Accrued interest payable	21,579	10,724
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	2,218,787	2,174,592
Interest-bearing deposits with banks	0	0
Resale agreements	0	0
Restricted equity securities, at cost	0	0
Loans held-for-sale	0	0
Loans held-for-investment, net	0	0
Branch assets held-for-sale		5,143
Mortgage servicing rights	0	0
Accrued interest receivable	0	0
Financial liabilities:
Demand, checking, savings and money market deposits	0	0
Time deposits	0	0
Branch liability held-for-sale		0
Short-term borrowings	0
FHLB advances	0	0
Repurchase agreements	0	0
Long-term debt	0	0
Accrued interest payable	0	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Cash and cash equivalents	0	0
Interest-bearing deposits with banks	400,900	398,422
Resale agreements	1,002,552	1,035,158
Restricted equity securities, at cost	73,729	73,521
Loans held-for-sale	3,114	85
Loans held-for-investment, net	0	0
Branch assets held-for-sale		10,970
Mortgage servicing rights	0	0
Accrued interest receivable	136,932	121,719
Financial liabilities:
Demand, checking, savings and money market deposits	24,928,338	25,974,314
Time deposits	8,730,602	5,626,855
Branch liability held-for-sale		0
Short-term borrowings	56,411
FHLB advances	335,800	335,901
Repurchase agreements	89,106	104,830
Long-term debt	162,566	171,673
Accrued interest payable	21,579	10,724
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Cash and cash equivalents	0	0
Interest-bearing deposits with banks	0	0
Resale agreements	0	0
Restricted equity securities, at cost	0	0
Loans held-for-sale	0	0
Loans held-for-investment, net	31,003,048	28,956,349
Branch assets held-for-sale		78,132
Mortgage servicing rights	12,077	11,324
Accrued interest receivable	0	0
Financial liabilities:
Demand, checking, savings and money market deposits	0	0
Time deposits	0	0
Branch liability held-for-sale		643,937
Short-term borrowings	0
FHLB advances	0	0
Repurchase agreements	0	0
Long-term debt	0	0
Accrued interest payable	$ 0	$ 0